Conventionally, in image forming processes carried out by electrophotography, electrostatic printing and so forth, the system is so set up that toner particles charged electrostatically develop electrostatic latent images on a photosensitive drum by the aid of an electrostatic force corresponding to potential differences produced on the photosensitive drum. Here, the toner is electrostatically charged by, stated specifically, the friction between toner particles themselves, between the toner and a carrier and further between the toner and a toner layer thickness control blade and so forth. Hence, it is essential to control, besides the particle diameter and particle size distribution of the toner, the charging performance (or chargeability) of the toner.
In order to control the charging performance of the toner, the triboelectric charge characteristics of a binder resin itself may be utilized. However, many binder resins used commonly in toners have low triboelectric charge characteristics, and it is not easy to control the charging performance by controlling their composition. Accordingly, it is common to add what is called a charge control agent capable of imparting charging performance to toners.
Conventionally, as negative-charging charge control agents, available are metal complex salts of monoazo dyes, metal compounds of nitrofumic acid and salts thereof, salicylic acid, alkylsalicylic acids, dialkylsalicylic acids, naphthoic acid, dicarboxylic acids and so forth, boron compounds, urea compounds, silicon compounds, carixarene, sulfonated copper phthalocyanine pigments, chlorinated paraffin, and so forth. Charge control agents containing these dyes or pigments are structurally complicated, are not constant in properties and have a poor stability. In particular, almost all of them cause changes in charging performance depending on environmental factors such as temperature and humidity. Also, some agents change in nature because of decomposition or the like at the time of heat kneading.
In addition, these charge control agents added to toners must be present on toner surfaces to a certain extent in order to impart triboelectric chargeability to the toners. Hence, such an additive may come off toner surfaces because of friction between toners themselves, their collision with a carrier, their friction with a transport sleeve or roller, a toner layer thickness control blade and a photosensitive drum to cause contamination of the carrier and so forth and contamination of developing members and the photosensitive drum. As the result, with an increase in the number of sheets during running, the charging performance becomes poor and at the same time any deterioration due to contamination also becomes worse to cause problems such as changes in image density and a lowering of image quality.
As stated above, charge control agents capable of imparting a sufficient charging performance to toners stably over a long period of time are seen to be very limited. Also, in order for them to be used in full-color toners, those to be added to the toners may preferably be colorless, and further, in order for them to be used in polymerization toners, may preferably have no polymerization inhibitory action. Therefore, taking account of these, only very few agents are feasible for practical use.
As the step of fixing toner images, it has been put forward to use, e.g., a pressure contact heating method making use of a heating roller (hereinafter “heating roller fixing method”), and a heat fixing method in which toner images are fixed bringing a fixing medium sheet into close contact with a heating element through a fixing film (hereinafter “film fixing method”).
In the heating roller or film fixing method, toner images held on the fixing medium sheet are made to pass the surface of the heating roller or fixing film while bringing the former into contact with the latter under application of pressure by means of a pressure member kept in touch with the latter. In this fixing method, since the surface of the heating roller or fixing film and the toner images held on the fixing medium sheet come into contact with each other under application of pressure, the heat efficiency in fusing the toner images onto the sheet is so high as to enable performance of rapid and good fixing.
In circumstances where electrophotographic apparatus used in recent years are variously demanded to be achievable of high image quality and to be made compact and light-weight, high-speed and high-productivity, energy-savable, highly reliable, low-cost, maintenance-free and so forth, it is an important technical subject how systems and materials are put forward which can achieve much higher speed, energy saving, high reliability and so forth, especially in the step of fixing. However, in order to resolve such a subject in the heating roller or film fixing method, it is essential, in particular, to vastly improve fixing performance of the toner that is a material. Thus, it is necessary to improve the performance of being capable of sufficient fixing to the fixing medium sheet at a lower temperature (hereinafter “low-temperature fixing performance”) and to improve the performance of being capable of prevention of offset which is a phenomenon in which contamination due to toner having adhered to the surface of the heating roller or film causes contamination of a next fixing medium sheet (hereinafter “anti-offset performance”).
In toners to be fixed by heat and pressure, toners incorporated with a wax having a high affinity for binder resins exhibits good anti-offset performance and low-temperature fixing performance under specific fixing conditions (see, e.g., Japanese Patent Application Laid-open Nos. H08-050367 and 2001-318484). In these toners, however, glass transition point of toner and melt viscosity of toner may lower as the wax blends with the binder resin, and hence, besides storage stability and fluidity, the charging performance tends to be damaged when it is aimed to further improve the low-temperature fixing performance, to tend to cause a great decrease in density and image defects especially when printed continuously. Accordingly, it is sought to provide a toner satisfying development stabilizing performance in virtue of superior charging performance and having further low-temperature fixing performance.
Now, as printers, LED or laser beam printers are prevalent in recent market, and techniques trend toward those having a higher resolution, that is, those which hitherto have a resolution of 300 or 400 dpi are being replaced by those having a resolution of 600 or 1,200 dpi. Accordingly, correspondingly thereto, developing systems have also come to be required to secure a higher minuteness. Also, in copying machines as well, they are being made more high-function, and hence are trending toward digital processing. This trend is chiefly a trend toward a method in which electrostatic latent images are formed using a laser, and hence the copying machines are also trending toward high resolution. Here is also demanded a high-resolution and high-minuteness developing system like the printers. As a means for meeting such a demand, toners are being made to have a smaller particle diameter, and toners with a small particle diameter and having specific particle size distribution are proposed (see, e.g., Japanese Patent Applications Laid-open No. H01-112253, No. H01-191156, No. H02-214156, No. H02-284158, No. H03-181952 and No. H04-162048).
However, as toners have smaller particle diameter, stable triboelectric charging of toner powder becomes an important technique. More specifically, the lowering of image stability as stated above tends to occur more remarkably unless fine individual toner particles are made to have a uniform charge quantity. This is because, as the toner merely has a small particle diameter, the toner particles adhere to the photosensitive member at a larger force (image force or van der Waals force) than Coulomb force acting on toner particles in the step of transfer, and, in addition thereto, since making toners have smaller particle diameter concurrently makes the toner have poor fluidity, the toner particles tend to have non-uniform charge quantity, so that toner particles causative of fog and having poor transfer performance may come present in a large number.
From the background as stated above, studies are energetically made on how toners can be improved in charge characteristics. In particular, on account of consideration for environment, requirement for stabler charging performance and reduction of production cost, it is proposed in recent years that a resin having a charge control function is used as a raw material for toners (see, e.g., Japanese Patent Publication No. H08-012467 and Japanese Patent No. 2663016).
According to what are disclosed in these publications, toners improved in charging performance are obtainable. However, as a result of studies made by the present inventors on these toners, it has turned out that they have a problem that, when printed on a large number of sheets, a toner charged to a reverse polarity increases gradually in a developing assembly to make what is called reversal fog rapidly come to occur seriously. Also, in these publications, disclosed is only a binary copolymer composed of styrene and 2-acrylamido-2-methylpropane sulfonic acid, and the binary copolymer disclosed therein has a glass transfer material temperature of 90° C. or more, and has caused a problem on the low-temperature fixing performance in some cases.
Toners containing a copolymer of a more improved, sulfonic acid group-containing acrylamide and a vinyl monomer (see, e.g., Japanese Patent Applications Laid-open No. H11-184165, No. H11-288129 and No. 2000-056518). However, in these publications as well, the achievement of both charging performance (in particular, rise performance at the initial stage) and fixing performance can not be well satisfactory.
In regard to the improvement in charging performance of the resin having a charge control function, too, some proposals are hitherto made (see, e.g., Japanese Patent No. 2807795 and Japanese Patent Application Laid-open No. H08-030017). According to what are disclosed in these publications, toners are obtainable which have a relatively good rise of charging and in which various additives are well stand dispersed in the binder resin. In regard to the charging performance, however, there is further room for improvement, and also these toners can not be said to be sufficient in regard to the low-temperature fixing performance as well. Further, in the toners disclosed in these publications, their transfer performance is not sufficient to cause problems on image quality in some cases.